Semaphorin3A Signaling Is Dispensable for Motor Axon Reinnervation of the Adult Neuromuscular Junction

Abstract

The neuromuscular junction (NMJ) is a specialized synapse that is formed by motor axon innervation of skeletal muscle fibers. The maintenance of motor-muscle connectivity is critical for the preservation of muscle tone and generation of movement. Injury can induce a robust regenerative response in motor axons, but severe trauma or chronic denervation resulting from neurodegenerative disease typically leads to inefficient repair and poor functional recovery. The axon guidance molecule Semaphorin3A (Sema3A) has been implicated as a negative regulator of motor innervation. Upon binding to a plexinA-neuropilin1 (Npn1) receptor complex, Sema3A initiates a downstream signaling cascade that results in axonal repulsion. Here, we established a reproducible nerve crush model to quantify motor nerve regeneration. We then used that model to investigate the role of Sema3A signaling at the adult NMJ. In contrast to previous findings, we found that Sema3A and Npn1 mRNA decrease in response to denervation, suggesting that Sema3A-Npn1 signaling may regulate NMJ reinnervation. To directly test that hypothesis, we used inducible knockout models to ubiquitously delete Sema3A or Npn1 from adult mice. Despite demonstrating that we could achieve highly efficient gene deletion, disruption of Sema3A-Npn1 signaling did not affect the normal maintenance of the NMJ or disrupt motor axon reinnervation after a denervating injury.

Significance Statement Increased axonal growth and sprouting after a peripheral nerve injury are thought to signify the activation of a robust regenerative response that results in more efficient recovery. The axon guidance factor Sema3A has been proposed to generate a growth inhibitory environment that reduces the regenerative capability of motor nerves. We sought to directly test how the inducible deletion of Sema3A or its receptor Npn1 alters muscle reinnervation after injury. Surprisingly, our work demonstrates that disruption of Npn1-Sema3A signaling does not alter reinnervation of the NMJ. Thus, the utility of targeting this pathway to improve recovery from denervating injuries may be more limited than suggested by earlier studies.